Hi.
Imagine a relay as a double switch. A small current from a dash switch for example, will switch the relay, which will then allow a large current to pass to whatever needs switching on.
If you look in the fusebox of your car you will see a few boxes, above the fuses, about an inch square of various colours. These are relays.
You probably can get them from maplins, and any other host of sources. Best bet would be a motor factors.
Good luck with that loom tho!!